This invention relates to optically pumped optical waveguide amplifiers, such as erbium doped optical fibre waveguide amplifiers. When such amplifiers are used in cascade in a transmission system for amplifying wavelength division multiplexed (WDM) signals, problems are liable to be encountered that arise from the effects of differential spectral gain. Such differential gain effects can, if unchecked, lead to signal corruption at one or more wavelengths because the signal power at these wavelengths has become too small in relation to the signal power at one or more of the other wavelengths. The problems associated with limiting differential gain effects are compounded by the fact that the spectral gain characteristic of one of these amplifiers alters shape with changes in operating conditions. These problems can in principle be avoided by arranging for some form of passive equalisation to take place using saturation effects in an optical amplifier. What is required of such an amplifier is that strong input signals shall, through the effects of saturation, experience less amplification than weaker input signals. It has been shown that such an effect can be produced in an erbium doped optical fibre amplifier by cooling it to about 77K so that the erbium transition shall be predominantly inhomogenously broadened. However, at temperatures nearer room temperature, homogeneous broadening predominates, with the result that the gain spectrum saturates relatively uniformly even though the optical power in only one of the WDM channels is increased significantly over that of the others.
An alternative approach is described by M N Zervas and R I Laming in an article entitled `Twin-Core Erbium Doped Channel Equalized`, Journal of Lightwave Technology, Vol 13, No 5, May 1985, pp 721-731. Ideally one would wish for -1 dB/dB equalisation so that for every 1 dB of power difference between one input signal and another there would be -1 dB of relative gain. The twin-ore approach does not achieve this target but is able, under fully optimised conditions to achieve in excess of -0.4 dB/dB. This is achieved by the action of the twin core in producing a partial separation between the regions where light of one wavelength propagating in the guide experiences gain, and the region where light of another wavelength experiences it. What happen is that light is launched into a single one of its twin cores thereby exciting both the zero order mode and the first order mode. For any given wavelength, these propagate at different velocities, and so at various positions along the guide the optical power is concentrated in the vicinity of one the cores, while at various intermediate positions it is concentrated in the vicinity of the other core. These positions are different for different wavelengths.